The present invention relates to a process for the production of purified pulp from lignocellulose-containing material. More particularly, the present invention relates to the production of pulp which has been purified in terms of removing harmful non-process compounds by an acidic pre-cleaning stage prior to delignification by alkaline cooking. Still more particularly, the present invention relates to a process for the production of a pulp to be bleached for papermaking pulp.
Throughout this disclosure, the term xe2x80x9calkaline cookingxe2x80x9d refers to pulp manufacturing processes well known in the art as kraft cooking, soda cooking and soda anthraquinone cooking.
All lignin-containing cellulosic materials in nature contain a wide variety of organic and inorganic compounds beside the main process compounds, lignin and cellulose. Unavoidably these non-process compounds enter the pulping process and will be subjected to the same chemical and physical treatment as the desired compounds. This is especially true in the case of alkaline delignification processes, such as kraft and soda cooking, which do not remove for example metal ions from the processed material. Traditionally, these non-process compounds have been led to the combustion and recovery line of the pulp mill with the spent liquor, or they have been ousted together with pulp mill effluents. Only some compounds have been separated and sold as by-products, such as sugars, tall oil and turpentine. Under the conventional circumstances of distribution of these compounds in a pulp mill process system, conventional pulping technology has been able to cope with the arising problems, such as foaming, deposits, higher consumption of bleaching chemicals, heavy metal chelating, just to mention a few from the huge list of routine difficulties in the operation.
Metals entering the process include all those occurring naturally in raw materials; monovalent metals sodium and potassium, earth-alkali divalent metals calcium, magnesium and barium, and heavy metals such as iron, copper and manganese. Under alkaline conditions metal ions are retained in the pulp and cause a lot of harm in terms of making the bleaching by oxygen chemicals, especially by hydrogen peroxide, less effective resulting in deteriorated pulp strength and excess chemical consumption. In addition divalent metals, especially calcium, tend to form precipitated deposits in process machinery, thus compromising operational efficiency. Currently, the metal problem is coped with by washing the metals to effluents after an acidic bleaching stage, or chelating metals in separate so called Q stages before peroxide bleaching stages. Once the metals are in the pulp mill cycle, they are difficult to remove. In practice the concentrations will increase to reach an equilibrium between dissolution and precipitation, and some precipitates become removed in the filtration of cooking liquors. It is quite clear that any process for removal of metals prior to their entering the pulp mill cycle would greatly improve the situation.
The side-groups in polysaccharides represent another group of non-process compounds. These side groups are not desired in the pulp product and their presence in the delignifying and bleaching processes is negative. It has been known for a long time that the acetyl groups of hemicelluloses are easily cleaved, but they consume alkali. Could they be removed prior to alkaline cooking, a lot of alkali could be saved for delignification. Another example is the formation of so-called hexenuronic acid groups from hemicellulose side-groups in alkaline cooking (Vuorinen et al., Selective hydrolysis of hexenuronic acid groups and its application in ECF and TCF bleaching of kraft pulps. International Pulp Bleaching Conference, Apr. 14-18, 1996, Washington D.C.). Downstream in the process, these groups are responsible for part of the bleaching chemical consumption and cause pulp brightness problems. Could they be removed prior to the alkaline cooking, the problem would be solved without any need for measures in the following process steps. In brief, the non-process compounds described above have negative effects in alkaline pulping. Metal ions are not removed due to the high pH, and polysaccharide side groups increase alkali consumption and react to form harmful compounds in the pulp. It does not make sense to introduce any unnecessary compounds into the alkaline cooking process; only the backbone polysaccharides are desired for the cellulosic pulp after delignification in cooking and bleaching.
As said above, under conventional conditions of non-process compound distribution in a pulp mill process system, the problems caused by these agents have been overcome by means of conventional pulping technology. However, contemporary pulping is developing in a very demanding direction: towards a closed-cycle pulp mill. Ultimately this means no effluents at all: the mill will recycle its own process water, which flows counter-currently to the pulping process. On the way towards zero-effluent pulp mill by reducing the wastewater amount, the industry has faced severe problems caused by the accumulation of non-process compounds in the processes. Various process internal measures and technologies have been proposed and applied to cope with undesired accumulating agents. Typical for all of these measures is that they are applied process-internally, i.e. in the middle of the fiber line, after the non-process compounds have entered the more or less closed process. It is quite obvious that the best remedy would be to prevent the non-process compounds to enter the fiber line, i.e. to remove them prior to the cooking stage, which is closely connected to the mill""s water cycle.
According to a technology called prehydrolysis kraft cooking, an acidic hydrolysis is carried out before delignification by kraft cooking (Rydholm, S. E., xe2x80x9cPulping Processesxe2x80x9d, Interscience, New York 1968, pp. 649 to 672; U.S. Pat. No. 5,589,033, Tikka). The objective of these processes is to remove as much hemicelluloses as possible from the cellulose macromolecule, which task the alkaline kraft cooking process can not accomplish. This is done in order to prepare pulp for products based on chemically modified cellulose such as viscose and cellulose acetate and other derivatives, which can not be manufactured in the presence of hemicelluloses. Although the prehydrolysis accomplishes a major cleaning effect, the resulting pulp has very low yield and is not suitable for papermaking purposes due to damaged fiber strength and the absence of hemicelluloses needed for fiber to fiber bonding in the paper web.
It has also been proposed to manufacture paper pulp after prehydrolysis for producing sugars to be used in fermentation to alcohols (U.S. Pat. No. 4,436,586, Elmore). According to this method, however, prehydrolysis conditions are strongly acidic and major pulp yield loss occurs due to the produced sugars. Unless used for sugar production this method cannot be an economical alternative for paper pulp production. It is also questionable, how well the paper technical properties can be maintained after such a loss of fiber bonding polysaccharides.
To remove the metals prior to alkaline cooking, chelation has been proposed (U.S. Pat. No. 5,593,544). This requires the use of chelating agents increasing the operational cost and introducing another group of organic compounds to be removed with the metals. In addition, chelation does not affect the polysaccharide side-groups hydrolytically as the pH is almost neutral, above 5.
One object of the present invention is to provide an improved alkaline delignification process for the preparation of pulp to be bleached for paper making, to be carried out within the framework of a modem closed-cycle pulp mill to meet present requirements for pulp purity after the cooking stage. In accordance with the present invention, these and other objectives have now been accomplished by means of a process for the production of pulp from lignin-containing cellulosic material, said process comprising an acidic precleaning stage for the removal of metals and side groups of polysaccharides, changing the process conditions of the cleaned lignocellulosic material from cleaning to alkaline delignification, and delignifying the precleaned lignocellulosic material with alkaline cooking liquor, yielding pulp suitable for bleaching to paper pulp. For the desired cleaning of the lignocellulosic material while retaining good pulp yield and paper-technical properties it is essential to adjust the acidic conditions to arrive at an only moderately acidic end-pH level of between about 2.5-5. A lower end-pH leads to the start of polysaccharide hydrolysis, resulting in severe yield losses and adverse changes in the paper-technical properties.
According to a preferred embodiment, the conditions for the precleaning are accomplished by steaming the lignocellulosic material in order to reach a desired temperature, preferably 100-140xc2x0 C., during a time sufficient for reaching an end-pH of about 2.5-5, preferably 3-4.
According to a second preferred embodiment, the conditions for the precleaning are accomplished by re-using steam on the lignocellulosic material in order to reach a desired temperature, preferably 100-140xc2x0 C., during a time sufficient for reaching an end-pH of about 2.5-5, preferably 3-4.
According to a third preferred embodiment, the conditions for the precleaning are accomplished by using water or, for example, clean condensate and reacting at a temperature between 40-150xc2x0 C. during a time sufficient for reaching an end-pH of 2.5-5, preferably 3-4.
According to a fourth preferred embodiment, the conditions for the precleaning are accomplished by using re-used precleaning liquid reacting at a temperature between 40-150xc2x0 C. for a time sufficient for reaching an end-pH of about 2.5-5, preferably 3-4.
According to a fifth preferred embodiment, the conditions for the precleaning are accomplished by using re-used precleaning liquid and adding an acidic chemical, then reacting at a temperature between 40-150xc2x0 C. for a time sufficient for reaching an end-pH of about 2.5-5, preferably 3-4.
According to a sixth preferred embodiment, the conditions for the precleaning are accomplished by using an acidic process liquid such as acidic bleaching filtrate or acidic condensate or wood room effluent, then reacting at temperature between 40-150xc2x0 C. a time sufficient for reaching an end-pH of about 2.5-5, preferably 3-4.
According to a seventh preferred embodiment, the transition from precleaning to alkaline delignification is carried out by introducing an alkaline process liquid and removing the portion of the resulting transition liquor that has a pH lower than 10. In this context, xe2x80x9calkaline process liquidxe2x80x9d means any available alkaline liquor, e.g white liquor, green liquor, spent alkaline cooking liquor or alkaline bleach plant filtrate.
According to an eighth preferred embodiment, the transition from the precleaning to alkaline delignification is carried out by introducing a washing liquid and subsequently removing the washing liquid by introducing the alkaline process liquid. In this context, washing liquid means any available aqueous medium, e.g water, condensate, or bleach plant filtrate.
In processes according to the invention, the lignocellulosic material is pre-cleaned prior to delignification in a more or less closed-cycle pulping process. In practice, metals and polysaccharide side groups attached to the fiber structures are transferred into the liquid medium surrounding the lignocellulosic material. Having been removed, these non-process compounds can be excluded from the process. Acidic or neutralized liquor from the transition stage before delignification can be conducted to the plant""s recovery facilities, where organic compounds will be combusted and metals will be removed as dregs and muds separated as white and green liquors are filtered before returned to the pulping process. In the pre-cleaning stage, metal ions are exchanged to protons; later, following the transition stage, the protons will be replaced by sodium which is the natural cation in the process. The amount of washing in the transition stage and the fate of the leaving liquors depend on the pulp mill in question and its liquor handling capacity. It is important to note that the different embodiments enable use of the present invention in a wide variety of situations, ranging from plants with overloaded evaporation/recovery facilities where only steam condensate and some neutralized alkaline liquor make up the removed volume, up to new plants which can be designed to handle larger washing liquid volumes in the transition stage. If process internal waste waters such as bleach plant filtrates and woodhandlling effluents are used, their treatment is simultaneously made more efficient.
The invention is applicable t o alkaline pulping processes as defined above, including processes operating batchwise or continuously. Batch processes include conventional as well as those employing the displacement method well known to those skilled in the art.